With the aggressive growth of battery-powered portable electronics, e.g., cell phones, the demand for lower cost, lighter weight and better efficiency battery chargers is very high. Historically, linear power supplies have been employed. However, despite being low in cost, linear power supplies cannot generally outperform switching mode power supplies, which have lower weight and much higher efficiency. For many applications, the flyback converter is often chosen from among different switching mode topologies to meet this demand due to its simplicity and good efficiency.
Over the years, various pulse width modulation (PWM) controller integrated circuit (IC) chips have been developed and used to build constant voltage flyback power supplies. Known designs require too many additional components to support the PWM controller IC chip, which increases cost and device size.
FIG. 1 (prior art) is a schematic diagram of an exemplary prior-art primary side controlled constant-output-voltage flyback converter circuit. Such a converter typically comprises a transformer 201 (which has three windings), a secondary side resistor 301 (which represents the copper loss of transformer 201), a primary switch 105, a secondary rectifier 302, an output capacitor 303, and a controller IC 104. A resistor 101 and a capacitor 102 provide the initial start-up energy for IC 104. Once the flyback converter is stable, IC 104 is powered by the auxiliary winding (with Na number of turns) of transformer 201 via rectifier 103. The output voltage is fed back to the primary side via the auxiliary winding, rectified and filtered by rectifier 107 and capacitor 110, and sensed by voltage divider resistors 108 and 109. A resistor 106 senses the current flowing through primary switch 105. IC 104 is a peak current mode PWM controller.
The circuit of FIG. 1 works well so long as the requirement of output voltage regulation is not stringent. Typically, 10% load regulation with a loading from 10% to 100% of its rated maximum load can be met. However, this regulation tends to become poor when loading drops below 10% of its rated load, both at least because the transformer copper loss varies with output current and input voltage and/or the auxiliary winding of transformer 201 contains an undesired resonant waveform when the flyback converter operates at discontinuous current mode (DCM).
In an attempt to meet this tight regulation requirement, the secondary side controlled flyback converter shown in FIG. 2 (prior art) is often used. Using this configuration, 5% or better load regulation with a loading from 10% to 100% of its rated maximum load can be typically achieved. In the circuit shown, the output voltage is sensed as an error signal by voltage divider resistors 305 and 307, and is monitored by a secondary IC 306. The error signal is then fed back to primary IC 104 via an optical coupler 202. A known disadvantage of this circuit, however, is its relatively high cost. For example, IC 306 and safety approved optical coupler 202 add significant cost, which can be up to 10% of the overall material cost in a typical application.
Some known approaches for primary feedback control of constant output voltage switching regulators teach the use of a reflected auxiliary winding voltage or current to control the peak voltage. One known deficiency of such known methods is that the constant control of the output voltage is applicable only in discontinuous conduction mode (DCM) operation, thereby limiting the power capability of the power converter. For continuous conduction mode (CCM) operation, current industry solutions almost exclusively rely on the use of an optocoupler as shown in FIG. 2. Typically, they will use the auxiliary current/voltage (e.g., via diode and RC filters) to control the peak primary voltage. When auxiliary voltage (i.e., the control voltage) decreases, the primary voltage is reduced. In addition, the output voltage variation versus load change and/or input voltage is often relatively poor. Thus, no tight regulation of input voltage is typically possible.
In view of the foregoing, what is needed is a relatively low-cost and effective control methodology of regulating the output voltage of a flyback converter. It would be desirable if at least some of the foregoing limitations of the prior art are overcome for both continuous voltage mode (CVM) and discontinuous mode (DCM) operation, preferably with a minimal number of IC chips (e.g., two IC chips). It is further desirable that the need for a secondary circuit and optical coupler are eliminated, and that the output voltage of a flyback converter be largely insensitive to temperature variations.